This invention relates to a process for producing Zn-Ni alloy plated steel strips.
Zn-Ni alloy plated steel strips are favorably evaluated as one of well-balanced automotive stocks because they are not only corrosion resistant, but also exhibit excellent properties required for automotive stocks including paintability, weldability, and workability.
Electro-galvanizing processes are most commonly used to deposit a Zn-Ni alloy layer on steel. Traditionally, the plating bath is a sulfate bath containing major proportions of zinc sulfate and nickel sulfate. Since the Ni anode is passivated and becomes insoluble in the sulfate bath, an insoluble Ni anode are used. Zn and Ni ions are replenished by individually dissolving Zn and Ni metals in water with the aid of suitable chemical agents to form make-up solutions outside the bath and adding the make-up solutions to the bath. This prior art process suffers from several problems.
(1) The mechanism of deposition of an alloy plating in the sulfate bath is abnormal codeposition in which Zn is preferentially deposited. In order to obtain a single .gamma. phase layer (nickel content 10-20%) having the best quality, the nickel molar ratio Ni/(Zn+Ni) in the bath should be increased up to as high as 0.60 to 0.70. The high concentration of expensive nickel increases the cost of bath formulation and the cost of make-up for a drag-out loss. (2) The concentration of Zn and Ni in the plating bath is gradually reduced as they are deposited onto the steel strip and lost by dragging out. To accommodate such concentration reduction, the bath must be frequently analyzed by means of a suitable analyzer capable of high precision analysis on line, for example, fluorescent X-ray analyzer for the purpose of making up chemicals or metals from outside the plating system. Bath maintenance is thus complicated and difficult.
(3) The insoluble anodes used are Pb alloys and Ti-Pt alloys which tend to deteriorate upon aging. Repair of such deteriorated anodes is expensive. In addition, dissolved-out anode materials contaminate the bath, and among others, lead is known to adversely affect the plating process. Lead in the bath may be filtered off by co-precipitating it with strontium carbonate although this process requires a large filter system and adds to a burden of associated operations like filter cleaning.
(4) The nickel content in a deposit should be consistent within a coil into which the plated strip is wound and between coils. Since the nickel content, however, tends to be affected by current density, line speed, and plating solution flow velocity, these operating parameters should be kept constant in every plating section in the electrogalvanizing line. The current density and line speed are difficult to keep them constant because they vary with strip width and deposit weight.
Since the alloy plating in sulfate bath has several problems as mentioned above, the inventors paid attention to the chloride bath which despite of poor deposit appearance, has only problems (1) and (3) among the above-mentioned problems (1) to (4) and presents the advantage of low electric power consumption due to increased conductivity. The result of this research is disclosed in U.S. patent application Ser. No. 568,189 filed on Dec. 30, 1983, which has solved the problems by using a plating solution having a composition as defined by the shaded region in FIG. 9. That is, a Zn-Ni alloy deposit of single .gamma.-phase (Ni 10 to 20%) having the best surface properties among Zn-Ni alloy deposits is obtained by preparing a plating solution having a composition within the region shown in FIG. 9.
The inventors encountered a problem in the use of the thus formulated plating solution. The anodes used in this chloride bath are soluble Zn and Ni anodes. The efficiencies of these anodes widely vary or are inconsistent. It is thus very difficult to set the currents introduced into the Zn and Ni anodes to optimum values and the current values are, in practice, adjusted through a trial-and-error or empirical procedure. During long term operation, the Ni and Zn concentrations of the bath deviate from the initial well-balanced relation. The resulting Zn-Ni alloy deposit become inconsistent in nickel content with the progress of plating, failing to always ensure the quality the users require.